1. Field of the Invention
The present invention relates generally to internal combustion engines and more particularly to combustion chamber intake valve arrangements, intake strokes and timing sequences therefor.
2. Description of the Related Art
Boosting the pressure of air introduced into an engine combustion chamber, by turbocharging for example, as a means of increasing engine shaft output has been known for many years. Turbocharging can also be used to improve fuel economy.
With turbocharging, the energy in engine exhaust gases is converted to power by the turbo machine's turbine. This power is mostly absorbed by the machine's compressor and reappears as boosted compressor outlet air pressure and increased mass airflow. Maximum compressor outlet air pressure is limited by compressor wheel design, generally in the range of a 2:1 pressure ratio between compressor outlet and compressor inlet pressures. At any given realized pressure ratio below this limit, the quantity of air delivered by the compressor is adjusted to correspond to the power supplied to it by the turbine. Assuming two stages of compression and the same available exhaust energy, the volume of air will be reduced in proportion to increased overall pressure ratio. Since two stages of compression can together provide nearly a 4:1 pressure ratio, air volume must be nearly halved.
In FIG. 1 the engine shaft output curve 20 and the turbo ideal power curve 22 cross at only one point. If the solid turbo power curve 22 of FIG. 1 represents best efficiency, the dashed curve 24 might represent five percent less efficient operation. The points 26, 28 at which the engine curve 20 and turbo curves 22, 24 cross are called the match points, and represent the powers and speeds at which the engine/turbo combinations should operate. Turbochargers are not constrained to operate along a fixed curve, but may have an infinite number of operating points within a range called a “map”. This map will show all of the possible operating points for a turbine or a compressor and will specify the operating efficiencies for these points.
Turbocharging has proven especially useful in steady state applications where engines need to run for long periods at or near their rated outputs at constant RPM. In the past, large diesel engines were routinely used for large powerplant and stationary purposes, such as pumping and electric generation, before being displaced by gas turbines. These large engine turbos could be matched at or near their best operating efficiencies since these engines operated at or near constant RPM/power settings. Turbochargers were also routinely used on these large engines to improve fuel efficiencies.
In other applications such as automotive, in which the engine speed and load typically varies considerably during operation, turbocharging has more limited value due to the difficulties of matching the characteristics of piston engines and turbochargers. Vehicle engines also work under widely varying load conditions, making it difficult to “match” a turbocharger to such an engine. The turbo can adapt to a range of engine operation conditions, but this range can be quite narrow. Consequently, automotive turbo use has been generally limited to applications with narrower power requirements (e.g., racing, tractor, or heavy truck applications) where it is easier to adapt to the turbo's limited range. In the case of regular vehicles such as passenger cars, turbos have been matched to the engine at less than full power output in order to mitigate turbo lag and provide a more useable combination. In the latter case, and often in the former, exhaust at full power must bypass the turbine to prevent damage from overspeeding the turbocharger or over-charging the engine cylinder. This bypass is accomplished through an exhaust “wastegate” which diverts at least a portion of the exhaust gases exiting the combustion chamber away from the turbine inlet, thereby limiting the power put into compressing the intake air.
FIG. 1 discussed above shows the generalized relationship between engine output and turbocharger reaction when these two devices are mated together. This figure is included to illustrate the real incompatibility between these two device types and why wastegates have traditionally been used to assist in matching and minimizing turbo lag problems. FIG. 1 illustrates the typical characteristics of a reciprocating engine, such as used in vehicles and other prime mover applications, and rotating machines such as aircraft gas turbines and of course, turbochargers. The rapidly rising engine power curve 20 is typical of the shape of horsepower and torque curves for reciprocating engines. As you step on the accelerator of your car, it moves out in direct response to this curve. When you reach the top of this curve, your car can no longer accelerate or travel faster. The discarded exhaust energy very much follows the same trend.
A turbine machine, if it were installed in your car, would react quite differently, if you were to “floor” the accelerator on a gas turbine, the whole machine would simply quit, described in gas turbine parlance as a “flame-out”. A gas turbine can supply air only by turning faster, and flooring the accelerator results in introducing far more fuel than can be burned with the air available at that instant. Had you depressed the accelerator only a little and waited for the machine to spin up, supplying more air, and followed that routine up to full throttle, you would likely have been successful in avoiding flameout. This process results in the curves 22, 24 of FIG. 1, which indicate turbo power. Turbochargers, being turbine-type machinery react similarly, except of course, they don't flame out. When you “floor” a turbocharged reciprocating engine, you initially get the full response of the engine only, with no turbo boost, since the turbo cannot respond quickly. After a few seconds, as the turbo begins to spin up, and the engine power increases more rapidly with greater turbine speed.
This inherent incompatibility between reciprocating engines and turbos results in so called “turbo lag” and other inter-operability difficulties. Turbo lag refers to the slow response of a turbocharger to sudden application of engine load. In a vehicle application, acceleration from a stop can create the sudden load change that leads to turbo lag. In present day applications, automotive engines are often equipped with two turbochargers to solve this problem. The smaller turbo machines in these applications have smaller rotating inertias, and consequently accelerate or “spool up” quicker and develop compressor pressures faster.
In a normal IC engine such as a diesel or spark ignition gasoline engine, cycle efficiency (and thus fuel consumption) is highly influenced by compression ratio. The higher the ratio, the better the efficiency. Diesels are known to be fuel efficient because they run at design compression ratio continuously. Spark ignition gasoline engines are throttled types and run at or near their design compression ratios only at wide open throttle. Gasoline engines in automotive use rarely run at wide open throttle, and consequently operate predominately at low and inefficient real compression ratios. Therefore, modern gasoline engines are designed with the highest possible mechanical or design compression ratios, knowing that these engines operate at wide open throttle and full compression ratio for only very short time periods. Spark ignition timing can be temporarily retarded for these short periods to prevent detonation while more normal throttle settings yield engine operation with somewhat improved (i.e., lower) realized compression ratios as a result.
One method used to improve Otto cycle efficiency is the Miller cycle in which inlet valve timing is modified to delay the closing of the intake valve until well into the compression stroke. During the compression stroke in the Miller cycle, air introduced into the combustion chamber during the preceding intake stroke is expelled back into the engine's intake manifold by the rising piston via the late-closing intake valve. Thus, actual compression work done by the piston, and performed only after the intake valve closes, occurs later in the compression stroke than in other Otto cycle engines. Since the amount of air trapped in the cylinder is also reduced by the late-closing intake valve of the Miller cycle, intake air pressure boosting with, for example, turbocharging, can replace this air loss without exceeding engine mechanical design limits.
Furthermore, supercharging or turbocharging high design ratio gasoline engines risks pushing the real or operational compression ratio beyond safe limits at wide open throttle. In practice, with turbocharging, compromise can be reached by lowering the design compression ratio and routing excess exhaust gases around the turbo with a wastegate, as described above.
An improved internal combustion cycle that promotes greater efficiencies than realized in prior engines, and avoids problems associated with cylinder overpressurization in engines where intake air pressure is substantially boosted relative to ambient pressure is highly desirable. Moreover, it would be desirable to facilitate such improvements in existing IC engines without extensive modification.